Dinuclear Mn Complexes Featuring Formal Metal–Metal Double Bonds

Dimanganese complexes
featuring formal metal–metal multiple
(MMM) bonds are scarce. Here, we examine the 32-e– bis-μ-phosphido complex [{Mn(CO)3(μ-PtBu2)}2] (1), which has
a formal bond order (fBO) of two. But does 1 actually
possess Mn–Mn multiple bonding? In a previous study, we answered
this question through assessment of its short Mn–Mn distance
and unusual low-energy absorption in the UV–vis spectrum. Herein,
we further address the question through sequential reduction, UV–vis,
and resonance Raman spectroscopic analysis, and computational methods.
Reduction of 1 resulted in several new complexes, some
with fBO = 2; only those with fBO = 2 exhibit the characteristic low-energy
MM→MM* transition predicted by TD-DFT, whereas species with
fBO = 1 do not. This work addresses the broader hypothesis that MMM
bonds in low-coordinate dimanganese complexes may act as spectators
or participants in reactions. This view aligns with prior observations
that CO dissociation from dimanganese carbonyls is required for alkyne
and H2 activation (Lacy; Abhyankar Chem. - Eur. J. 2023, e202300518). These results
suggest that the elusive Mn–Mn multiple bond is more than just
an esoteric curiosity but a plausible hidden participant in the landscape
of dinuclear manganese catalysis.